The long-term objectives of this laboratory are to define the genetic causes and elucidate the biochemical and metabolic effects of inherited deficiency of the purine metabolic enzyme adenosine deaminase (ADA). This knowledge is essential for understanding the basis of, and developing effective treatment for, the clinical manifestations of ADA deficiency, primarily Severe Combined Immunodeficiency Disease (SCID), a fatal disorder in infants (ADA deficiency accounts for 15-20% of all cases of SCID). However, about a fifth of patients with ADA deficiency develop immune deficiency more insidiously, leading to diagnosis later in childhood, adolescence, or adulthood. One of this laboratory's main aims has been to characterize the spectrum of ADA gene mutations in patients with different degrees of clinical severity, and to systematically define the effects of specific mutations on the structure, function, and cellular expression of ADA. This research is necessary to establish how a patient's genetic makeup (genotype) affects clinical severity (phenotype), and it may provide a basis for predicting prognosis and possibly the response to enzyme replacement or gene therapy. We have identified an important subset of mutations that interferes with the folding of ADA into an active structure. We will now examine the ability of chaperone proteins found in the cytoplasm of lymphoid cells to enhance the expression of these "folding" mutations, which may modulate the severity of immune deficiency. Extracellular ADA may regulate effects of adenosine on lymphocyte function. We have defined the binding site of ADA for the cell membrane protein CD26, and provided evidence that ADA/CD26 interaction is not essential for immune function in humans. We will now investigate the direct interaction of ADA receptors that mediate signal transduction by adenosine, using a sensitive biochemical method for detecting ADA binding to membranes prepared from cells that express specific human adenosine receptor subtypes (but not CD26 or intracellular ADA). We will also characterize the catalytic activity and role in the metabolism of adenosine and adenosine analogs of the CECR1 protein, which is the human ortholog of a class of insect secreted "adenosine deaminase related growth factors" postulated to control extracellular levels of adenosine.